The long-term objective is to define the cellular mechanisms by which an insect peptide hormone (prothoracicotropic hormone, or PTTH) stimulates the secretion of a developmentally important insect steroid (ecdysone). Ecdysone is produced by the prothoracic glands, paired organs located in the thorax. For reasons of tissue homogeneity, manipulability, and large cell size, the prothoracic glands provide a unique model for investigating cellular mechanisms of peptide hormone action. PTTH has recently been shown to activate the prothoracic glands of the tobacco hornworm, Manduca sexta, via two intracellular messenger molecules -- cyclic AMP (cAMP) and calcium. It appears that calcium enhances the synthesis of cAMP, and that the cyclic nucleotide in turn stimulates steroidogenesis through the action of a cAMP-dependent protein kinase. Direct evidence will be obtained regarding: a) the regulation of cAMP synthesis and degradation by PTTH; b) the involvement of cAMP-dependent protein kinase and protein phosphorylation in PTTH action; c) the effects of PTTH on intracellular free calcium levels; and d) the effects of calcium and calmodulin on cAMP metabolism. Emphasis has intentionally been placed on early steps in glandular activation (as opposed to steps specifically related to ecdysone synthesis) as information obtained at this level should prove applicable to processes of stimulus-secretion coupling in a variety of insect and vertebrate cells.